1. Field of the Invention
The present invention relates to a lens holding member or lens barrel for use in an optical device, and more particularly to a lens holding structure for holding a lens system in a lens holding member or a lens barrel for use in an optical instrument.
2. Description of the Relevant Art
Optical instruments with lenses, such as cameras, binoculars, collimators, or the like are required to support the lenses accurately in a desired position. To meet this requirement, the optical devices have a lens barrel or a lens holding member with reference portions for engaging the lenses. When the lenses, which are usually circular in shape, are supported in the lens barrel, they are accurately positioned when their circumferential edges are engaged by the reference portions of the lens barrel. Typically, a lens holding member for a camera comprises a portion of the body of the camera or the lens barrel of a camera lens unit, and a lens holding member for a binocular comprises the lens barrel of the binocular. Each reference portion for engagement with a lens may comprise a shoulder on the inner wall surface of a lens barrel and an inner circumferential surface, adjacent to the shoulder, of the lens mount. When the circumferential surface of the lens on one side thereof is engaged by the shoulder, the lens is positioned along its optical axis (i.e., in the axial direction of the lens barrel). When the outer circumferential edge of the lens is engaged by the inner circumferential surface of the lens barrel, the optical axis of the lens is aligned with the central axis of the lens barrel. The lens which is thus positioned as desired in the lens barrel is fixed to the lens barrel by either a retainer ring that is force-fitted or threaded into the lens barrel or a plurality of screws. The retainer ring or screws are held in direct engagement with the lens, thereby rigidly holding the lens in the lens barrel.
Precision optical devices such as high-quality cameras, for example, include lens barrels made of a metallic material such as an aluminum alloy. The most general material of lenses is glass. The glass material has a thermal expansion coefficient which is not largely different from that of the metallic material. Therefore, when a glass lens is supported in a metallic lens barrel, any difference between the thermal expansion coefficients of the lens and the lens barrel does not adversely affect the rigid lens holding structure at considerably high or low temperatures. This is because the lens remains appropriately supported in the lens barrel due to slight elastic deformation of the lens mount or retainer ring.
In recent years, lenses of synthetic resin have been widely used because of their advantages resulting from reduced weight and high productivity. Since, however, the thermal expansion coefficient of synthetic resin lenses is much greater than those of metallic materials, it is difficult to support a synthetic resin lens rigidly in a metallic lens barrel. More specifically, both diameter and thickness of the synthetic resin lens increase at higher temperatures. The increase in the lens diameter may be absorbed by a clearance between the outer circumferential edge of the lens and the inner circumferential surface of the lens barrel. Alternatively, an elastic spacer may be disposed between the outer circumferential surface of the lens and the inner circumferential surface of the lens barrel, as disclosed in U.S. patent application Ser. No. 07/580,467 filed Sep. 11, 1990 which is entitled "STRUCTURE FOR HOLDING AN OPTICAL ARTICLE" and assigned to the same assignee as the assignee of the present application, now abandoned. With the above arrangements, the lens holding structure is completely free from problems which would otherwise be caused by temperature-dependent changes in the lens diameter.
However, temperature-dependent changes in the lens thickness are liable to cause the lens to be appreciably shifted in position in the axial direction. If the lens is supported using the aforesaid rigid lens holding structure, then any large thermal expansion of the synthetic resin lens at higher temperature forces the lens and the metallic lens barrel to be deformed beyond elastically deformable limitations, resulting in plastic deformations or permanent deformations of the lens and/or the lens mount. Once the lens and/or lens barrel has been permanently deformed, the lens is no longer firmly supported in the lens barrel when the temperature drops again, so that images which are formed by the lens tend to be poor in quality.
Several efforts to solve the problems resulting from temperature-dependent changes in the lens thickness have been proposed so far. For example, Japanese Laid-Open Utility Model Publication No. 62(1987)-164305 discloses a lens holding structure by which a plastic lens is supported in a lens barrel. In the disclosed lens holding structure, the circumferential surface of the plastic lens on one side thereof is held against a distal end surface of the lens barrel, thus positioning the lens along the optical axis thereof, and the plastic lens is resiliently pressed against the distal end surface of the lens mount by a sheet-like elastic member. Temperature-dependent changes in the lens thickness are absorbed by the sheet-like elastic member. However, the disclosed lens holding structure can only be used with the lens at the distal end of the lens mount, and is not applicable to the positioning of two lenses which are disposed close to each other.
Lens systems which comprise a plurality of lenses which are closely arranged and supported in a single lens mount (e.g., interchangeable lenses for high quality cameras) have very small gaps or clearances between the lenses. With such strict space limitations, the axial expansion of the synthetic resin lenses due to temperature changes poses a greater problem.